The southeastern area of the brazilian continental margim: its evolution during the Middle and Late Cretaceous as indicated by paleoecological data

Abstract

Biostratigraphic and paleoecologic investigations of Albian through Maastrichtian sediments in the Santos and Campos basins made possible a preliminary reconstruction of the paleoenvironmental evolution of the southeastern Brazilian continental margin. Five major Albian-Maastrichtian depositional sequences have been recognized in the Campos and Santos basins, and their paleoenvironmental maps drawn. As result, it was possible to reconstruct the regional paleogeography of the area at the close of each sequence. This procedure allowed inferences concerning sea-level variations in the Southeastern Brazilian Continental Margin. The oldest marine fossil record in the Campos and Santos basins is found in thick Lower/Middle Albian carbonate layers. These sediments were deposited in a shallow neritic environment with hypersaline and oxygen-rich bottom waters. The Upper Albian sequence consists on marls and calcilutites deposited under deeper bathymetric conditions , mostly mid to deep neritic. Such relative sea-level rise is of global extent and contributed to lowering the salinity of the waters then covering the southeastern portion of the Brazilian continental margin. The Upper Albian carbonatic sequence is overlain by and Upper Cenomanian/Middle Turonian siliciclastic sequence with varied faunal content, interpreted as result of environmental fluctuations. This sequence records anoxic events and turbidite deposition interlay in normal marine deposits, environmental conditions ranging from deep neritic to upper bathyal. The depositional history of the two basins evolved separately from the Late Turonian through the Maastrichtian. The Santos Basin was subject to strong sedimentary influx that led to the development of marginal and shelf facies. In its tum, continuous sinkage of the Campos Basin allowed oceanic conditions to develop in the studied area during this time. The sedimentary record for this phase includes two sequences, separated by an intervening unconfonnity due to the lack of Campanian deposits.